The present invention relates to an enciphering/deciphering system for transforming data into an encoded message before transmission thereof and for transforming a received message back into unscrambled data.
For example, the enciphering/deciphering systems GC-201, GC-505 and GC-515 of the company Gretag AG, Regensdorf, Switzerland are typical representatives of such enciphering/deciphering systems. These systems contain an enciphering/deciphering generator as a principal item, whose structure and initial states are determined, among other things, by a primary key, or code, and a non-secret, randomly determined auxiliary code. With each new initialization of the system (first transmission reception, resumption after disruption, etc.), a new randomly determined auxiliary code is automatically generated, transmitted and loaded into the enciphering/deciphering generators on the transmission and reception ends of the system. On the other hand, as a general rule, the secret primary code is changed only after relatively long time intervals.
In the case of older systems, the primary code change takes place by means of direct input of the new code into the generator with a keyboard or the like. The above-named GC-505 and GC-515 systems have a memory on the transmitting and receiving ends, in which a number (e.g. 30) of identical transmission and reception side secret primary codes are kept available. To change the primary code, in the case of the GC-515 system, the memory address or the number of the desired code need be put in only at each station via a selection switch or the like, and the charging or loading of the code into the encipher generator then takes place automatically. Code changes (e.g. to the next highest code number) are undertaken after certain time intervals agreed upon between the partner stations. In the case of the GC-505 system, it is sufficient to select the code address at the calling station, after which it is transmitted automatically to the partner station.
In the case of ciphered transmission, for reasons of safety, it is known that the amount of data (i.e. number of bits) transmitted for a certain code setting must be limited. An upper limit is given by the structure-caused recursion length of the cipher generators to be used. The maximum recursion time, during which one may work with an unchanged code setting, depends on this recursion length and is naturally inversely proportional to the transmission rate. For example, a weekly change of the primary code is recommended for the above-named GC-515 system at a transmission rate of 19.2 kb/s. Since the code memory of the GC-515 system contains a total of 30 codes, the memory would have to be replaced every six to seven months if the system is operated continuously. This relatively frequent changing of the primary code and the frequent replacement of the code memory cause an undesirable administrative and personnel effort which, following the trend to higher transmission rates, may become greater in the future.